1. Field of the Invention
This invention relates to high speed, high density electrical connectors for use in transmitting high frequency signals and more particularly to matched impedance, low crosstalk connectors suitable for use in minimizing transmission delays.
2. Description of the Prior Art
As the requirements for increased speed in electronic equipment, such as computers, becomes ever more stringent, the limiting factor appears to be the actual signal transmission time in the various signal lines that must be employed in computer systems. As the speeds at which computer systems are required to operate continues to increase, the circuit density has to be concurrently expanded. These increases in speed and density cannot be achieved without suitable electrical connectors employed between printed circuit boards, wires, and other transmission lines employed to interconnect various components of computer systems.
Electrical connectors almost inevitably introduce mechanical and electrical discontinuities in transmission circuits. At high frequencies, the discontinuities introduced by electrical connectors can serve to significantly reduce the signal transmission speed. Abrupt changes in the shape of conductive paths or dielectric materials, which are virtually inevitable upon introduction of electrical connectors, can result in a change in the characteristic impedance of the conductive path. These changes in conductive diameters represent discontinuities which behave as capacitances shunting the conductive paths at each point in a connector where the diameter change occurs.
An ideal connector for interconnecting separate components of a transmission path would be at least as good with respect to signal distortion and energy loss as that of the physical components comprising the particular transmission path. Ideally, a given connector should be physically identical to an incremental length of the transmission path to satisfy this requirement. However, exact conformity is difficult if not impossible to achieve due in part to such considerations as the mechanical integrity of the interconnection and the suitability of various dielectric materials for use in a connector. The geometric mismatch of the connector, as compared to the transmission path, creates an impedance mismatch. The impedance mismatch in turn creates an energy reflection. When a signal passing along a line of one impedance encounters a section of a line of different impedance, for example due to geometric mismatch, there is a reflection of a portion of the signal where the impedance changes. The greater the frequency, generally the greater the reflected signal. Furthermore, the length of the mismatched line in conjunction of the frequency of the signal is significant. If the length of the mismatch is less than one half a wave length, the effect of the impedance mismatch tends not to be as significant. For higher frequencies, however, impedance mismatching can become a significant problem.
The effect of impedance mismatching in coaxial connectors is discussed in greater detail in U.S. Pat. No. 3,350,666, U.S. Pat. No. 3,460,072 and U.S. Pat. No. 3,651,432. Those discussions of the effect of impedance mismatching are incorporated herein by reference. Impedance mismatches can result in significant signal distortion as well as potential propagation delay, thus affecting the performance of transmission paths. Controlled matched impedance connectors are, therefore, necessary for high speed signal transmission in such applications as large high speed computers.
It is possible to provide compensation for impedance mismatching. One such compensation technique involves the introduction of compensating impedance variations adjacent an impedance mismatch caused by the presence of an electrical connector. U.S. Pat. No. 3,323,083; U.S. Pat. No. 3,350,666; U.S. Pat. No. 3,460,072; and U.S. Pat. No. 4,389,625, each disclose impedance compensation techniques involving changes in the impedance over various lengths of a transmission path to compensate for the mismatch impedance which occurs in a coaxial connector. Other techniques which have been employed in matched impedance connectors involve the introduction of a compensating impedance, such as a compensating capacitance. For example, additional capacitance can be added by introducing additional grounding surfaces. One example of a connector which adds a compensating capacitance by the addition of conductive pads common to a ground plane is that shown in U.S. Pat. No. 3,651,432. Strip line or microstrip connectors rely upon a continuous ground plane to maintain a constant impedance. U.S. Pat. Nos. 3,871,728 and 4,223,968 are examples of matched impedance printed circuit board connectors employing additional ground planes. Although connectors of this type may exhibit a substantially constant impedance, the crosstalk between signal lines can be quite significant. Furthermore, these connectors do not provide shielding from extraneous radiation, such as electromagnetic interface (EMI) and radio frequency interference (RFI).
Of course, in other connectors, alternating ground and signal configurations, similar to those employed in high speed transmission cables, are continued in the connector. It will be appreciated, however, that the addition of ground paths in the connector itself substantially reduces the density which may be attained in a given connector.
U.S. Pat. No. 4,451,107, incorporated herein by reference, discloses a connector having a die cast housing, dielectric sleeves, and die cast terminals within the dielectric sleeve. The die cast housing is formed of zinc, which acts as a shield to attenuate unwanted electromagnetic interference and to provide a high speed connector without the use of grounding terminals. In that connector, the dielectric sleeves are molded into apertures in the die cast housing. The terminals are subsequently die cast into openings formed in the dielectric sleeves. Modular components can then be mounted on printed circuit boards to provide the desired impedance characteristics through the length of the connector assembly. The connector disclosed therein does, however, have certain drawbacks. The use of die cast terminals has proven unsatisfactory, due in part to the metallurgy of the die cast materials, such as zinc, used to form the terminals. Furthermore, the right angle configuration disclosed therein inevitably creates certain impedance variations at the terminal bend. The suitability of that approach to high density connectors is also limited, due to the difficulty of die casting the extremely thin walls between adjacent terminals.